Used boolean modifier for lettering on 3D print. It won't print letters, but shows as working
Try with file https://drive.google.com/file/d/0B7t43CPM6VA-RURVZDg0NGhuTEk/view?usp=sharing (I'll delete it from my drive at Oct 28).

I did the following: 


Converted text to mesh. 
Combined all text meshes together. 
Extruded text meshes on Z axis. 
Did the boolean modifier to carve the plate. 
Applied the modifier to the plate. 
Deleted the text meshes. 
Exported to STL.